Converter lens, interchangeable lens, and image pickup apparatus

ABSTRACT

A converter lens includes three or more negative lenses and increases the focal length of an entire system, in which an average refractive index Ndave at the d-line (wavelength of 587.56 nm) of a material of all the negative lenses included in the converter lens is defined.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. application Ser. No.16/443,657, filed Jun. 17, 2019, which claims priority from JapanesePatent Application No. 2018-121364 filed Jun. 26, 2018, which are herebyincorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION Field of the Invention

The aspect of the embodiments relates to a converter lens, aninterchangeable lens, and an image pickup apparatus.

Description of the Related Art

There is known a rear converter lens (denoted as converter lens below)capable of increasing the focal length of an entire system when beingarranged between an interchangeable lens and an image pickup apparatus.

US2017/0277022 discloses a converter lens including five negative lensesand a focal length enlarging magnification of 2.0.

Generally, a converter lens has a negative refractive power and thenegative refractive power tends to increase along with an increase infocal length enlarging magnification. Further, there has been known thatwhen the curvature of a negative lens in a converter lens is increasedin order to increase the negative refractive power, aberrations such ascoma aberration due to off-axis light easily occur.

Further, the converter lens does not include an aperture stop, and thusan off-axis light passing through a master lens in an interchangeablelens enters an image plane although its principal light does not crosswith an optical axis of the converter lens. Aberration correction cannotbe made by the lenses arranged before and after the aperture stop likethe interchangeable lens, and thus the aberration correction is likelyto be difficult to make by the converter lens.

SUMMARY OF THE INVENTION

According to the aspect of the embodiments, a converter lens has anegative refractive power and increases a focal length of an entiresystem when arranged on an image side of a master lens. The converterlens consists of: a first lens unit having positive refractive power;and a second lens unit having negative refractive power and arranged onan image side of the first lens unit, in which the first lens unitconsists of a negative lens and a first positive lens, three or morenegative lenses are included in the converter lens, and the followingconditional equation is satisfied:

1.92<Ndave<2.10.

where Ndave represents an average refractive index at a d-line of amaterial of all negative lenses included in the converter lens.

Further features of the disclosure will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-section view of a converter lens according to a firstembodiment.

FIG. 1B is a cross-section view illustrating the converter lensaccording to the first embodiment arranged on an image side of a masterlens.

FIG. 2 is a longitudinal aberration diagram at the time of in-focus onan infinite object when the converter lens according to the firstembodiment is arranged on the image side of the master lens.

FIG. 3 is a lateral aberration diagram at the time of in-focus on aninfinite object when the converter lens according to the firstembodiment is arranged on the image side of the master lens.

FIG. 4A is a cross-section view of a converter lens according to asecond embodiment.

FIG. 4B is a cross-section view of the converter lens according to thesecond embodiment arranged on the image side of the master lens.

FIG. 5 is a longitudinal aberration diagram at the time of in-focus onan infinite object when the converter lens according to the secondembodiment is arranged on the image side of the master lens.

FIG. 6 is a lateral aberration diagram at the time of in-focus on aninfinite object when the converter lens according to the secondembodiment is arranged on the image side of the master lens.

FIG. 7A is a cross-section view of a converter lens according to a thirdembodiment.

FIG. 7B is a cross-section view of the converter lens according to thethird embodiment arranged on the image side of the master lens.

FIG. 8 is a longitudinal aberration diagram at the time of in-focus onan infinite object when the converter lens according to the thirdembodiment is arranged on the image side of the master lens.

FIG. 9 is a lateral aberration diagram at the time of in-focus on aninfinite object when the converter lens according to the thirdembodiment is arranged on the image side of the master lens.

FIG. 10A is a cross-section view of a converter lens according to afourth embodiment.

FIG. 10B is a cross-section view of the converter lens according to thefourth embodiment arranged on the image side of the master lens.

FIG. 11 is a longitudinal aberration diagram at the time of in-focus onan infinite object when the converter lens according to the fourthembodiment is arranged on the image side of the master lens.

FIG. 12 is a lateral aberration diagram at the time of in-focus on aninfinite object when the converter lens according to the fourthembodiment is arranged on the image side of the master lens.

FIG. 13A is a cross-section view of a converter lens according to afifth embodiment.

FIG. 13B is a cross-section view of the converter lens according to thefifth embodiment arranged on the image side of the master lens.

FIG. 14 is a longitudinal aberration diagram at the time of in-focus onan infinite object when the converter lens according to the fifthembodiment is arranged on the image side of the master lens.

FIG. 15 is a lateral aberration diagram at the time of in-focus on aninfinite object when the converter lens according to the fifthembodiment is arranged on the image side of the master lens.

FIG. 16A is a cross-section view of a converter lens according to asixth embodiment.

FIG. 16B is a cross-section view of the converter lens according to thesixth embodiment arranged on the image side of the master lens.

FIG. 17 is a longitudinal aberration diagram at the time of in-focus onan infinite object when the converter lens according to the sixthembodiment is arranged on the image side of the master lens.

FIG. 18 is a lateral aberration diagram at the time of in-focus on aninfinite object when the converter lens according to the sixthembodiment is arranged on the image side of the master lens.

FIG. 19 is a cross-section view of the master lens.

FIG. 20 is a longitudinal aberration diagram of the master lens at thetime of in-focus on an infinite object.

FIG. 21 is a lateral aberration diagram of the master lens at the timeof in-focus on an infinite object.

FIG. 22A is a perspective view illustrating a configuration of an imagepickup apparatus.

FIG. 22B is a side view illustrating the configuration of the imagepickup apparatus.

DESCRIPTION OF THE EMBODIMENTS

Converter lenses according to embodiments of the disclosure and an imagepickup apparatus will be described below in detail with reference to theattached drawings.

When the refractive indexes of d-line (587.56 nm), F-line (486.13 nm),and C-line (656.27 nm) in the Fraunhofer lines are respectivelyrepresented as Nd, NF, and NC, the Abbe constant vd of a material isexpressed as follows:

vd=(Nd−1)/(NF−NC).

A converter lens according to each embodiment is arranged between animage pickup apparatus and an interchangeable lens detachable from theimage pickup apparatus, for example. A converter lens according to eachembodiment can further increase the focal length of an imaging opticalsystem (entire system) consisting of an optical system of theinterchangeable lens and the converter lens than the focal length whenthe imaging optical system is configured of only the interchangeablelens.

The left side is an object side (front) and the right side is an imageside (rear) in the cross-section views of the converter lens illustratedin FIGS. 1, 4, 7, 10, 13, and 16 and the cross-section view of themaster lens illustrated in FIG. 19. Li represents i-th lens unit when“i” is an order of lens unit from the object side toward the image sidein each cross-section view An aperture stop SP determines (limits) abeam of full aperture F-number (Fno). FP indicates a flare-cut stop forcutting undesirable lights.

When the image pickup apparatus is a digital video camera or a digitalcamera, an image plane IP corresponds to an imaging device(photoelectric conversion device) such as CCD sensor or CMOS sensor.When the image pickup apparatus is a silver-halide film camera, theimage plane IP corresponds to a film surface.

FIGS. 2, 5, 8, 11, 14, and 17 are longitudinal aberration diagrams ofthe converter lenses according to the embodiments described below,respectively, and FIG. 20 is a longitudinal aberration diagram of amaster lens. In the spherical aberration diagrams, the solid lineindicates the d-line and the two-dot chain line indicates the g-line.The broken line M indicates a meridional image plane and the solid lineS indicates a sagittal image plane in the astigmatism diagrams.Distortion aberration is indicated for the d-line. Magnificationchromatic aberration is indicated for the g-line. ω indicates a halfangle of view (degrees) and Fno indicates an F-number.

FIGS. 3, 6, 9, 12, 15, and 18 are lateral aberration diagrams of theconverter lenses according to the embodiments described below,respectively, and FIG. 21 is a lateral aberration diagram of the masterlens. In the lateral aberration diagrams, the broken line M indicatesaberration on the meridional image plane, and the solid line S indicatesaberration on the sagittal image plane.

As described above, in a converter lens entirely having a negativerefractive power, aberrations such as coma aberration due to off-axislight easily occur and is likely to be difficult to correct.

Thus, the converter lenses according to the embodiments entirely have anegative refractive power and include three or more negative lenses.Then, the average refractive index of a material of the negative lensesincluded in a converter lens is made relatively higher thereby todecrease the curvature of each lens surface and to restrict anoccurrence of aberrations such as coma aberration due to off-axis light.

Specifically, when Ndave represents the average refractive index at thed-line (wavelength of 587.56 nm) of the material of all the negativelenses included in the converter lens, the following conditionalequation is satisfied:

1.92<Ndave<2.10  (1).

When the average refractive index of the material of the lenses lowersthe lower limit of the conditional equation (1), the Petzval sum can beset to be small, and the image curvature and the like are easy tocorrect. However, undesirably the curvatures of the surfaces of thelenses increase, and aberrations such as coma aberration are difficultto correct.

When the average refractive index of the material increases, thedispersion of the glass material generally increases. Thus, when theaverage refractive index of the material of the negative lenses ishigher than the upper limit of the conditional equation (1), undesirablymagnification chromatic aberration is difficult to correct.

In this way, the converter lenses according to the embodiments meet theabove lens configuration and the conditional equation (1), and thusaberration due to off-axis light such as coma aberration can becorrected, and high optical performance can be obtained also when theconverter lenses are mounted on the master lens.

Further, the converter lenses according to the present embodiments areused so that the master lens for an image pickup apparatus including animaging device with a low maximum image height can be used by a userwithout a feeling of strangeness in terms of aberration even when it isused for an image pickup apparatus including an imaging device with ahigh maximum image height.

In one embodiment, the numerical range of the conditional equation (1)is set as follows:

1.95<Ndave<2.08  (1a).

Further, the numerical range of the conditional equation (1) is set asfollows:

1.98<Ndave<2.05  (1b).

Further, a converter lens may include three or more negative lenses andmore preferably four or more negative lenses in a second lens unit.

Further, a converter lens meets one or more of the following conditionalequations:

1.30<(R1+R2)/(R1−R2)<2.50  (2)

0.10<sk/TD<0.50  (3)

−2.30<f1/f2<−0.95  (4)

−1.40<f1/f<−0.30  (5).

The curvature radius of the surface on the object side of the lensarranged closest to the image side is represented as R1, and thecurvature radius of the surface on the image side of the lens isrepresented as R2. The air-converted length from the surface of theconverter lens closest to the image side to the image plane when theconverter lens is arranged on the image side of the master lens isrepresented as sk, and the length on an optical axis from the surface ofthe converter lens closest to the object side to the surface closest tothe image side is represented as TD. The focal length of a first lensunit is represented as f1, the focal length of a second lens unit isrepresented as f2, and the focal length of the converter lens isrepresented as f. Here, the first lens unit consists of one negativelens and one positive lens. The conditional equation (2) defines a shapeof the positive lens in the converter lens arranged closest to the imageside. In one embodiment, the curvature radius of one surface is largerthan that of the other surface in order to largely refract an off-axislight and to restrict an occurrence of aberration. Thereby, an off-axislight can be further refracted than an on-axis light and aberrations dueto an off-axis light can be easily corrected while an occurrence ofaberration is restricted.

The conditional equation (2) is defined in terms of the above points.When a difference between the curvature of the surface on the image sideand the curvature of the surface on the object side is larger to bebelow the lower limit of the conditional equation (2), undesirably thefield curvature enters under-correction. When a difference between thecurvature of the surface on the image side and the curvature of thesurface on the object side is smaller to be over the upper limit of theconditional equation (2), undesirably the field curvature entersover-correction.

The conditional equation (3) defines a ratio of backfocus of theconverter lens relative to the length (lens structure length) from thesurface on the object side of the lens arranged closest to the objectside to the surface on the image side arranged closest to the imageside. Undesirably the lens structure length is longer when theconditional equation (3) is lowered. When the lens structure length isshorter to be over the conditional equation (3), undesirably therefractive power of each lens is higher and spherical aberration isdifficult to correct.

The conditional equation (4) defines a ratio of the focal length of thefirst lens unit relative to the focal length of the second lens unit.When the lower limit of the conditional equation (4) is lowered,undesirably spherical aberration largely occurs to be over-correctionand is difficult to correct. When the upper limit of the conditionalequation (4) is exceeded, undesirably spherical aberration largelyoccurs to be under-correction and is difficult to correct.

The conditional equation (5) defines a ratio of the focal length of thefirst lens unit relative to the focal length of the converter lens. Whenthe absolute value of the focal length of the first lens unit is largerto be below the lower limit of the conditional equation (5) and therefractive power is lower, undesirably spherical aberration occurs to beover-correction. When the absolute value of the focal length of thefirst lens unit is lower to be over the upper limit of the conditionalequation (5) and the refractive power is higher, undesirably sphericalaberration occurs to be under-correction.

Further, the numerical ranges of the conditional equations (2) to (5)are set as follows:

1.40<(R1+R2)/(R1−R2)<2.30  (2a)

0.15<sk/TD<0.40  (3a)

−2.10<f1/f2<−1.00  (4a)

−1.20<f1/f<−0.32  (5a).

Further, the numerical ranges of the conditional equations (2) to (5)are set as follows:

1.50<(R1+R2)/(R1−R2)<2.10  (2b)

0.25<sk/TD<0.35  (3b)

−1.90<f1/f2<−1.10  (4b)

−1.10<f1/f<−0.34  (5b).

At least one of the above conditional equations is satisfied, higheroptical performance can be obtained in the entire system even when theconverter lens is arranged on the image side of the master lens.

In one embodiment, the first lens unit consists of a cemented lens inwhich one negative lens and one positive lens are bonded. Thereby, anoccurrence of chromatic aberration can be restricted.

The master lens according to an embodiment and the converter lensesaccording to the embodiments will be described below.

[Converter Lens]

The converter lenses according to the first to sixth embodiments will bedescribed below.

First Embodiment

FIG. 1A is a cross-section view of a converter lens RCL according to thefirst embodiment. FIG. 1B is a cross-section view of a master lens ML,and the converter lens RCL according to the first embodiment arranged onthe image side of the master lens ML. FIG. 2 and FIG. 3 are alongitudinal aberration diagram and a lateral aberration diagram,respectively, at the time of in-focus on an infinite object when theconverter lens RCL according to the first embodiment is arranged on theimage side of the master lens ML. The enlarging magnification of theconverter lens RCL is 1.61.

A first lens unit is configured of a cemented lens of a negative lens G1and a positive lens G2.

A second lens unit consists of a cemented lens consisting of a negativelens, a positive lens, and a negative lens, and a positive lens arrangedon the image side of the cemented lens. That is, the converter lens RCLincludes three negative lenses.

Second Embodiment

FIG. 4A is a cross-section view of a converter lens RCL according to thesecond embodiment. FIG. 4B is a cross-section view of the master lens MLand the converter lens RCL according to the second embodiment arrangedon the image side of the master lens ML. FIG. 5 and FIG. 6 are alongitudinal aberration diagram and a lateral aberration diagram,respectively, at the time of in-focus on an infinite object when theconverter lens RCL according to the second embodiment is arranged on theimage side of the master lens ML. The enlarging magnification of theconverter lens RCL is 1.60.

A first lens unit is configured of a cemented lens of a negative lens G1and a positive lens G2.

A second lens unit consists of a cemented lens consisting of a negativelens and a positive lens, a negative lens, a positive lens, a negativelens, and a positive lens arranged from the object side toward the imageside in this order. That is, the converter lens RCL includes fournegative lenses.

Third Embodiment

FIG. 7A is a cross-section view of a converter lens RCL according to thethird embodiment. FIG. 7B is a cross-section view of the master lens MLand the converter lens RCL according to the third embodiment arranged onthe image side of the master lens ML. FIG. 8 and FIG. 9 are alongitudinal aberration diagram and a lateral aberration diagram,respectively, at the time of in-focus on an infinite object when theconverter lens RCL according to the third embodiment is arranged on theimage side of the master lens ML. The enlarging magnification of theconverter lens RCL is 2.01.

The first lens unit is configured of a cemented lens of a negative lensG1 and a positive lens G2.

The second lens unit consists of a cemented lens consisting of anegative lens and a positive lens, a negative lens, a positive lens, anegative lens, and a positive lens arranged from the object side towardthe image side in this order. That is, the converter lens RCL includesfour negative lenses.

Fourth Embodiment

FIG. 10A is a cross-section view of a converter lens RCL according tothe fourth embodiment. FIG. 10B is a cross-section view of the masterlens ML and the converter lens RCL according to the fourth embodimentarranged on the image side of the master lens ML. FIG. 11 and FIG. 12are a longitudinal aberration diagram and a lateral aberration diagram,respectively, at the time of in-focus on an infinite object when theconverter lens RCL according to the fourth embodiment is arranged on theimage side of the master lens ML. The enlarging magnification of theconverter lens RCL is 2.02.

The first lens unit is configured of a cemented lens of a negative lensG1 and a positive lens G2.

The second lens unit consists of a cemented lens consisting of anegative lens and a positive lens, a negative lens, a negative lens, apositive lens, a negative lens, and a positive lens arranged from theobject side toward the image side in this order. That is, the converterlens RCL includes five negative lenses.

Fifth Embodiment

FIG. 13A is a cross-section view of a converter lens RCL according tothe fifth embodiment. FIG. 13B is a cross-section view of the masterlens ML and the converter lens RCL according to the fifth embodimentarranged on the image side of the master lens ML. FIG. 14 and FIG. 15are a longitudinal aberration diagram and a lateral aberration diagram,respectively, at the time of in-focus on an infinite object when theconverter lens according to the fifth embodiment is arranged on theimage side of the master lens ML. The enlarging magnification of theconverter lens RCL is 1.61.

The first lens unit is configured of a cemented lens of a negative lensG1 and a positive lens G2.

The second lens unit consists of a cemented lens consisting of anegative lens, a positive lens, and a negative lens, and a positive lensarranged on the image side of the cemented lens. That is, the converterlens RCL includes three negative lenses.

Sixth Embodiment

FIG. 16A is a cross-section view of a converter lens RCL according tothe sixth embodiment. FIG. 16B is a cross-section view of the masterlens ML and the converter lens RCL according to the sixth embodimentarranged on the image side of the master lens ML. FIG. 17 and FIG. 18are a longitudinal aberration diagram and a lateral aberration diagram,respectively, at the time of in-focus on an infinite object when theconverter lens RCL according to the sixth embodiment is arranged on theimage side of the master lens ML. The enlarging magnification of theconverter lens RCL is 1.59.

The first lens unit is configured of a cemented lens of a negative lensG1 and a positive lens G2.

The second lens unit consists of a cemented lens consisting of anegative lens, a positive lens, and a negative lens, and a positive lensarranged on the image side of the cemented lens. That is, the converterlens RCL includes three negative lenses.

[Master Lens]

FIG. 19 is a cross-section view of the master lens ML at the time ofin-focus on an infinite object. FIG. 20 is a longitudinal aberrationdiagram of the master lens ML at the time of in-focus on an infiniteobject. FIG. 21 is a lateral aberration diagram of the master lens ML atthe time of in-focus on an infinite object.

The master lens ML is a fixed focal length lens consisting of anaperture stop SP, a front lens group Lf arranged on the object side ofthe aperture stop SP, and a rear lens group Lr arranged on the imageside of the aperture stop SP. The F-number of the master lens ML is2.88, and the half angle of view thereof is 29 degrees. Theabove-described master lens ML is exemplary, and any other opticalsystem capable of forming an image on the image plane may be employed.

Numerical Embodiments

A numerical embodiment of the master lens ML, and first to sixthnumerical embodiments corresponding to the converter lenses RCLaccording to the first to sixth embodiments, respectively, will bedescribed.

In each numerical embodiment, a surface number indicates an order of anoptical surface from the object side. The definition of referencesymbols are as described above, where r indicates a curvature radius(mm) of an optical surface, d at a surface number i indicates aninterval (mm) between an i-th optical surface and an i+1-th opticalsurface, nd indicates a refractive index of a material of an opticalmember at the d-line, and vd indicates an Abbe number of a material ofan optical member with respect to the d-line.

BF indicates backfocus. Backfocus denotes a length on an optical axisfrom the surface closest to the image side to a paraxial image plane inair-converted length.

A lens full length of the master lens ML is a length in which a lengthon an optical axis from a surface (first lens surface) of the masterlens ML closest to the object side to a surface (final lens surface) ofthe master lens ML closest to the image side is added with backfocus. Alens full length when a converter lens RCL is arranged on the image sideof the master lens ML is a length in which a length on an optical axisfrom the surface of the master lens ML closest to the object side to thesurface of the converter lens RCL closest to the image side is addedwith backfocus of the converter lens RCL.

A lens interval between the master lens and the converter lens is alength on an optical axis from the surface of the master lens closest tothe image side to the surface of the converter lens closest to theobject side. The interval between the master lens and the converter lensis denoted in air-converted length. An enlarging magnification is aratio of the focal length of the entire system when the master lens andthe converter lens are employed relative to the focal length of themaster lens.

A lens structure length of the converter lens is a length on an opticalaxis from the surface of the converter lens closest to the object sideto the surface of the converter lens closest to the image side.

An effective aperture is an aperture in a range in which an on-axislight and an off-axis light pass. An incident pupil position is a lengthfrom the surface closest to the object side to the incident pupil, andan exit pupil position is a length from the surface closest to the imageside to the exit pupil. A front principal point position is a lengthfrom the surface closest to the object side to the front principalpoint, and a rear principal point position is a length from the surfaceclosest to the image side to the rear principal point. Each numericalvalue of the front principal point position and the rear principal pointposition is the paraxial amount, and its symbol assumes a direction fromthe object side toward the image side as positive.

When the optical surface is aspherical, a symbol * is denoted on theright of a surface number. Assuming x for the displacement amount from asurface vertex in an optical axis direction, h for the height from anoptical axis in a direction vertical to the optical axis, R for aparaxial curvature radius, k for a conic constant, and A4, A6, A8, A10,and A12 for an aspherical coefficient of each order, an aspherical shapeis expressed as follows:

x=(h ² /R)/[1+{1−(1+k)(h/R)²}^(1/2) +A4×h ⁴ +A6×h ⁶ +A8×h ⁸ +A10×h ¹⁰+A12×h ¹².

“e±XX” at each aspherical coefficient indicates “×10±^(XX).” Thephysical amounts used in the abovementioned conditional equationsaccording to the first to sixth numerical embodiments are indicated in[Table 1], and the values corresponding to the respective conditionalequations are indicated in [Table 2].

The lengths in the following numerical embodiments are expressed in mmand the angles are expressed in degrees, but the lengths may beexpressed in other unit since the optical system can be used to beproportionally increased or proportionally decreased.

[Master Lens]— for all of First to Sixth Numerical Embodiments ofConverter Lenses

In mm

Surface data Surface number r d nd vd Effective aperture 1 23.706 2.831.91082 35.3 16.13 2 63.184 0.25 14.42 3 22.266 0.90 1.48749 70.2 12.894 7.213 4.00 10.11 5 ∞ 3.26 8.13(Flare-cut stop) 6(Stop) ∞ 3.24 8.78 7−16.321 4.82 1.69680 55.5 9.30 8 −8.400 0.80 1.80610 33.3 11.37 9−36.438 0.20 13.67 10  227.537 4.39 1.59522 67.7 15.23 11  −15.547 0.9016.72 12* −34.842 3.55 1.58313 59.4 17.76 13  −15.035 35.68 18.94 Imageplane ∞ Aspherical data 12-th surface K = 0.00000e+000 A 4 =−5.24174e−005 A 6 = 5.25723e−008 A 8 = −3.53661e−009 A10 = 3.36031e−011A12 = −1.48386e−013 Various items of data of master lens Focal length24.50 F-number 2.88 Half angle of view (degrees) 29.14 Image height13.66 Lens full length 64.83 BF 35.68 Incident pupil position 10.34 Exitpupil position −35.77 Front principal point position 26.44 Rearprincipal point position 11.18 Single lens data of master lens LensStart surface Focal length 1 1 40.28 2 3 −22.32 3 7 19.87 4 8 −13.72 510 24.62 6 12 42.54

[Converter Lens]

First Numerical Embodiment

In mm

Surface data Surface number r d nd vd Effective aperture 1 141.314 1.201.95375 32.3 25.80 2 20.811 8.90 1.80518 25.4 25.40 3 −50.545 4.45 25.604 −36.637 1.20 2.00100 29.1 24.10 5 62.836 8.70 1.62588 35.7 25.00 6−19.857 1.30 2.00100 29.1 25.80 7 −424.225 7.20 29.00 8 −74.271 9.351.59551 39.2 35.20 9 −25.409 13.96 37.90 Image plane ∞ Interval betweenmaster lens and converter lens 4.19 Various items of data when converterlens is arranged on image side of master lens Focal length 39.54F-number 4.65 Half angle of view (degrees) 28.68 Image height 21.64 Lensfull length 89.59 BF 13.96 Various items of data of converter lens Focallength −148.00 Lens structure length 42.30 Front principal pointposition −24.78 Rear principal point position −76.86 Enlargingmagnification 1.61 Single lens data of converter lens Lens Start surfaceFocal length 1 1 −25.71 2 2 19.39 3 4 −22.98 4 5 25.13 5 6 −20.84 6 860.53

Second Numerical Embodiment

In mm

Surface data Surface number r d nd vd Effective aperture 1 108.446 1.202.00100 29.1 25.90 2 18.934 8.60 1.85478 24.8 25.40 3 −62.820 4.00 25.404 −49.683 1.20 2.05090 26.9 24.30 5 27.755 6.00 1.80810 22.8 24.90 6−109.942 1.30 25.50 7 −45.352 1.20 2.05090 26.9 25.60 8 144.322 0.1027.30 9 49.805 5.00 1.53172 48.8 29.50 10 −100.714 1.30 30.30 11 −52.5341.60 2.00100 29.1 30.40 12 −149.361 2.90 32.10 13 −86.192 8.00 1.5407247.2 34.30 14 −27.523 13.08 36.50 Image plane ∞ Interval between masterlens and converter lens 4.19 Various items of data when converter lensis arranged on image side of master lens Focal length 39.21 F-number4.61 Half angle of view (degrees) 28.89 Image height 21.64 Lens fulllength 88.80 BF 13.08 Various items of data of converter lens Focallength −83.09 Lens structure length 42.40 Front principal point position0.34 Rear principal point position −36.78 Enlarging magnification 1.60Single lens data of converter lens Lens Start surface Focal length 1 1−23.07 2 2 17.89 3 4 −16.81 4 5 27.97 5 7 −32.73 6 9 63.41 7 11 −81.63 813 71.36

Third Numerical Embodiment

In mm

Surface data Surface number r d nd vd Effective aperture 1 71.603 1.202.05090 26.9 25.50 2 17.334 9.10 1.85478 24.8 24.20 3 −55.856 0.45 24.004 −83.837 1.20 2.00100 29.1 23.40 5 15.257 8.15 1.80810 22.8 22.50 6−109.942 3.45 22.80 7 −36.477 1.20 2.05090 26.9 22.80 8 53.517 0.2024.30 9 38.586 4.70 1.53172 48.8 26.20 10 −174.517 6.95 27.20 11−127.684 1.60 2.00100 29.1 32.50 12 424.763 4.65 33.50 13 −73.668 10.001.51742 52.4 35.20 14 −24.017 11.00 37.90 Image plane ∞ Interval betweenmaster lens and converter lens 4.19 Various items of data when converterlens is arranged on image side of master lens Focal length 49.27F-number 5.79 Half angle of view (degrees) 23.71 Image height 21.64 Lensfull length 97.18 BF 11.00 Various items of data of converter lens Focallength −81.18 Lens structure length 52.85 Front principal point position−9.30 Rear principal point position −71.02 Enlarging magnification 2.01Single lens data of converter lens Lens Start surface Focal length 1 1−22.01 2 2 16.42 3 4 −12.82 4 5 17.08 5 7 −20.50 6 9 59.89 7 11 −97.93 813 64.44

Fourth Numerical Embodiment

In mm

Surface data Surface number r d nd vd Effective aperture 1 81.547 1.202.05090 26.9 25.60 2 17.175 8.90 1.85478 24.8 24.40 3 −74.877 0.40 24.104 −800.041 1.20 2.00100 29.1 23.60 5 15.215 8.10 1.80810 22.8 22.40 6−109.942 1.35 22.50 7 −59.136 1.20 2.00100 29.1 22.40 8 800.004 3.9522.80 9 −35.142 1.20 2.05090 26.9 23.40 10 309.433 0.15 25.30 11 50.3205.10 1.53172 48.8 28.20 12 −77.229 0.60 29.10 13 −79.630 1.60 2.0010029.1 29.20 14 387.478 6.50 30.70 15 −116.685 10.00 1.51742 52.4 35.50 16−25.328 13.96 37.90 Image plane ∞ Interval between master lens andconverter lens 4.19 Various items of data when converter lens isarranged on image side of master lens Focal length 49.40 F-number 5.81Half angle of view (degrees) 23.65 Image height 21.64 Lens full length98.73 BF 13.96 Various items of data of converter lens Focal length−75.46 Lens structure length 51.45 Front principal point position −6.52Rear principal point position −62.68 Enlarging magnification 2.02 Singlelens data of converter lens Lens Start surface Focal length 1 1 −20.90 22 17.11 3 4 −14.91 4 5 17.03 5 7 −54.97 6 9 −29.98 7 11 58.11 8 13−65.88 9 15 60.27

Fifth Numerical Embodiment

In mm

Surface data Surface number r d nd vd Effective aperture 1 105.372 1.101.90525 35.0 25.80 2 20.274 9.30 1.74077 27.8 25.40 3 −43.036 3.30 25.504 −34.856 1.20 1.85150 40.8 24.20 5 50.808 9.00 1.54072 47.2 24.90 6−19.532 1.30 2.00330 28.3 25.60 7 −612.486 7.20 28.90 8 −70.239 9.601.61293 37.0 35.20 9 −24.676 13.95 37.90 Image plane ∞ Interval betweenmaster lens and converter lens 4.19 Various items of data when converterlens is arranged on image side of master lens Focal length 39.45F-number 4.64 Half angle of view (degrees) 28.74 Image height 21.64 Lensfull length 89.28 BF 13.95 Various items of data of converter lens Focallength −162.19 Lens structure length 42.00 Front principal pointposition −29.93 Rear principal point position −84.93 Enlargingmagnification 1.61 Single lens data of converter lens Lens Start surfaceFocal length 1 1 −27.90 2 2 19.84 3 4 −24.12 4 5 27.32 5 6 −20.13 6 857.46

Sixth Numerical Embodiment

In mm

Surface data Surface number r d nd vd Effective aperture 1 258.268 1.002.00100 29.1 25.80 2 23.518 7.70 1.85478 24.8 25.70 3 −54.781 6.10 25.904 −30.390 1.00 2.05090 26.9 24.40 5 31.048 7.40 1.89286 20.4 26.30 6−48.829 1.00 2.24163 16.9 27.30 7 −1396.349 8.60 28.70 8 −130.600 9.801.51742 52.4 36.30 9 −26.447 13.65 38.60 Image height ∞ Interval betweenmaster lens and converter lens 4.19 Various items of data when converterlens is arranged on image side of master lens Focal length 39.02F-number 4.59 Half angle of view (degrees) 29.00 Image height 21.64 Lensfull length 89.58 BF 13.65 Various items of data of converter lens Focallength −194.07 Lens structure length 42.60 Front principal pointposition −40.70 Rear principal point position −101.32 Enlargingmagnification 1.59 Single lens data of converter lens Lens Start surfaceFocal length 1 1 −25.90 2 2 20.16 3 4 −14.49 4 5 22.23 5 6 −40.77 6 862.10

TABLE 1 First Second Third Fourth Fifth Sixth Numerical numericalnumerical numerical numerical numerical numerical value embodimentembodiment embodiment embodiment embodiment embodiment Ndave 1.985252.02595 2.02595 2.02090 1.92002 2.09784 R1 −74.271 −86.192 −73.668−116.685 −70.239 −130.600 R2 −25.409 −27.523 −24.017 −25.328 −24.676−26.447 sk 13.961 13.078 11.000 13.958 13.949 13.651 TD 42.300 42.40052.850 51.450 42.000 42.600 f1 64.383 66.738 54.431 78.133 56.824 74.656f2 −52.306 −38.060 −36.198 −41.299 −50.216 −65.021 f −148.003 −83.095−81.181 −75.460 −162.193 −194.067

TABLE 2 First Second Third Fourth Fifth Sixth Conditional numericalnumerical numerical numerical numerical numerical equation embodimentembodiment embodiment embodiment embodiment embodiment (1) Ndave 1.985252.02595 2.02595 2.02090 1.92002 2.09784 (2) (R1 + R2)/(R1 − R2) 2.0401.938 1.967 1.554 2.083 1.508 (3) sk/TD 0.330 0.308 0.208 0.271 0.3320.320 (4) f1/f2 −1.231 −1.754 −1.504 −1.892 −1.132 −1.148 (5) f1/f−0.435 −0.803 −0.670 −1.035 −0.350 −0.385

Embodiment of Image Pickup Apparatus

FIG. 22 is diagrams illustrating a configuration of an image pickupapparatus (digital camera) 10. FIG. 22A is a perspective view and FIG.22B is a side view. The image pickup apparatus 10 includes a camera mainbody 13, the master lens ML, a converter lens RCL similar to thataccording to any one of the above first to sixth embodiments, and alight receiving device (imaging device) 12 for photoelectricallyconverting an image formed by the master lens ML and the converter lensRCL. The light receiving device 12 can employ an imaging device such asCCD sensor or CMOS sensor. The master lens ML and the converter lens RCLmay be integrally configured with the camera main body 13, or may beconfigured to be detachable from the camera main body 13, respectively.

When the master lens ML and the converter lens RCL are integrallyconfigured with the camera main body 13, the converter lens RCL isconfigured to be able to be inserted on an optical axis.

Embodiment of Interchangeable Lens

The aspect of the embodiments is applicable to an interchangeable lensin which the master lens ML and the converter lens RCL are configured inthe same barrel and which is detachable from the image pickup apparatus.The interchangeable lens may be a fixed focal length lens with fixedfocal length, or may be a zoom lens with variable focal length. In thiscase, the converter lens RCL is configured to be able to be inserted onan optical axis. The converter lens RCL is arranged on an optical axisor off an optical axis in response to a user instruction via anoperation member or user interface.

Exemplary embodiments of the disclosure have been described above, butthe disclosure is not limited to the embodiments and examples, and canbe variously combined, modified, and change within the scope of thespirit.

While the disclosure has been described with reference to exemplaryembodiments, it is to be understood that the disclosure is not limitedto the disclosed exemplary embodiments. The scope of the followingclaims is to be accorded the broadest interpretation so as to encompassall such modifications and equivalent structures and functions.

What is claimed is:
 1. A converter lens which has a negative refractivepower and increases a focal length of an entire system when arranged onan image side of a master lens, the converter lens consisting of: afirst lens unit having positive refractive power; and a second lens unithaving negative refractive power and arranged on an image side of thefirst lens unit, wherein the first lens unit consists of a negative lensand a first positive lens, wherein three negative lenses are included inthe converter lens, and wherein the following inequalities aresatisfied:1.92<Ndave<2.10,−1.40<f1/f≥−0.670, and0.10<sk/TD<0.35, where Ndave represents an average refractive index at ad-line of a material of the three negative lenses included in theconverter lens, f1 represents a focal length of the first lens unit, frepresents a focal length of the converter lens, sk represents anair-converted length from a surface of the converter lens closest to theimage side to an image plane while the converter lens is arranged on theimage side of the master lens, and TD represents a length on an opticalaxis from a surface of the converter lens closest to the object side toa surface closest to the image side.
 2. The converter lens according toclaim 1, wherein the second lens unit including a second positive lensarranged closest to an image side of the converter lens, wherein thefollowing inequality is satisfied:1.30<(R1+R2)/(R1−R2)<2.50, where R1 represents a curvature radius of asurface of the second positive lens on an object side and R2 representsa curvature radius of a surface of the second positive lens on the imageside.
 3. The converter lens according to claim 1, wherein the negativelens and the first positive lens are cemented to each other.
 4. Theconverter lens according to claim 1, wherein the following inequality issatisfied:−2.30<f1/f2<−0.95, where f1 represents a focal length of the first lensunit and f2 represents a focal length of the second lens unit.
 5. Theconverter lens according to claim 1, wherein the second lens unitincludes three or more negative lenses.
 6. The converter lens accordingto claim 1, wherein the second lens unit comprises four or more negativelenses.
 7. An interchangeable lens comprising: a master lens; and aconverter lens which has a negative refractive power and increases afocal length of an entire system when arranged on an image side of themaster lens, wherein the converter lens consists of a first lens unithaving positive refractive power and second lens unit having negativerefractive power and arranged on an image side of the first lens unit,wherein the first lens unit consists of a negative lens and a firstpositive lens, wherein three negative lenses are included in theconverter lens, and wherein the following inequalities is satisfied:1.92<Ndave<2.10,−1.40<f1/f≥−0.670, and0.10<sk/TD<0.35, where Ndave represents an average refractive index atthe d-line of a material of the three negative lenses included in theconverter lens, f1 represents a focal length of the first lens unit, frepresents a focal length of the converter lens, sk represents anair-converted length from a surface of the converter lens closest to theimage side to an image plane while the converter lens is arranged on theimage side of the master lens, and TD represents a length on an opticalaxis from a surface of the converter lens closest to the object side toa surface closest to the image side.
 8. The interchangeable lensaccording to claim 7, wherein the second lens unit including a secondpositive lens arranged closest to an image side of the converter lens,wherein the following inequality is satisfied:1.30<(R1+R2)/(R1−R2)<2.50, where R1 represents a curvature radius of asurface of the second positive lens on an object side and R2 representsa curvature radius of a surface of the second positive lens on the imageside.
 9. The interchangeable lens according to claim 7, wherein thenegative lens and the first positive lens are cemented to each other.10. The interchangeable lens according to claim 7, wherein the followinginequality is satisfied:—2.30<f1/f2<−0.95, where f1 represents a focal length of the first lensunit and f2 represents a focal length of the second lens unit.
 11. Theinterchangeable lens according to claim 7, wherein the followinginequality is satisfied:−1.40<f1/f<−0.30, where f1 represents a focal length of the first lensunit and f represents a focal length of the converter lens.
 12. Anapparatus comprising: a master lens; a converter lens which has anegative refractive power and increases a focal length of an entiresystem when arranged on an image side of the master lens; and an imagingdevice configured to receive an image formed by the master lens and theconverter lens, wherein the converter lens consists of a first lens unithaving positive refractive power and second lens unit having negativerefractive power and arranged on an image side of the first lens unit,wherein the first lens unit consists of a negative lens and a firstpositive lens, wherein three negative lenses are included in theconverter lens, and wherein the following inequalities is satisfied:1.92<Ndave<2.10,−1.40<f1/f≥−0.670, and0.10<sk/TD<0.35, where Ndave represents an average refractive index atthe d-line of a material of the three negative lenses included in theconverter lens, f1 represents a focal length of the first lens unit, frepresents a focal length of the converter lens, sk represents anair-converted length from a surface of the converter lens closest to theimage side to an image plane while the converter lens is arranged on theimage side of the master lens, and TD represents a length on an opticalaxis from a surface of the converter lens closest to the object side toa surface closest to the image side.
 13. The apparatus according toclaim 12, wherein the second lens unit including a second positive lensarranged closest to an image side of the converter lens, wherein thefollowing inequality is satisfied:1.30<(R1+R2)/(R1−R2)<2.50, where R1 represents a curvature radius of asurface of the second positive lens on an object side and R2 representsa curvature radius of a surface of the second positive lens on the imageside.
 14. The apparatus according to claim 12, wherein the negative lensand the first positive lens are cemented to each other.
 15. Theapparatus according to claim 12, wherein the following inequality issatisfied:−2.30<f1/f2<−0.95, where f1 represents a focal length of the first lensunit and f2 represents a focal length of the second lens unit.
 16. Theapparatus according to claim 12, wherein the following inequality issatisfied:−1.40<f1/f<−0.30, where f1 represents a focal length of the first lensunit and f represents a focal length of the converter lens.